Skewed Rotor Designs for Hybrid Homopolar Electrical Machines

ABSTRACT

Provided is a method for implementing skewing in a hybrid homopolar generator comprising. The method includes aligning inductor poles within an axial front segment of a rotor, with corresponding magnets within an axial back segment of the rotor. The method also includes moving, during assembly, the axial front segment and the axial back segment relative to each other such that inductor poles in the axial front segment and the axial back segment form a pattern.

I. TECHNICAL FIELD

The present disclosure relates generally to electrical machines. Inparticular, the present disclosure relates to reducing flux harmonicsbetween a rotor and a stator in a hybrid homopolar (HHP) electricalmachine.

II. BACKGROUND

HHP electrical machines, are adaptable for use in avionics generally,and aircraft engines in particular. These generators, which represent anintegration of traditional electrical power generation techniques, aretypically lighter and more efficient than conventional generators, thusmaking them suitable for use in the avionics industry.

A HHP generator is an embellishment of a type of machine referred to inthe art as a homopolar inductor alternator. In homopolar inductoralternators, a direct current (DC) excitation field coil and alternatingcurrent (AC) armature coils are situated in the stator. The armaturecoils must be linked with alternating flux, obtained by magneticreluctance variations embodied in the rotor structure.

As a practical matter, a typical HHP stator assembly includes two(split) stator halves aligned axially, corresponding to respectiveaxially aligned rotor sections. The unidirectional field coil issituated between the two stator halves. The reluctance variationsembodied in the rotor act upon the unidirectional flux produced by thefield coil to produce an alternating flux seen by the armature coils. Asunderstood in the art, the term hybrid implies inclusion of magnets inthe rotor. The nature of the magnet approach will also produce analternating flux seen by the armature coils.

A major challenge of conventional HHP generators is stator tooth fluxripple: an effect causing armature voltage variations that interact withthe armature load, the armature winding, and with the rotor. Oneconsequence of this interaction is a creation of flux harmonic losses onthe rotor, in particular on the rotor sleeve. These harmonic losses canimpose a pulsating force on the rotor, causing the rotor to overheat.These losses are described in greater detail below in terms of idealvoltage and torque waveforms.

Rotor loss reduction, in pursuit of an ideal voltage waveform, andslot-order space harmonic reduction, to achieve an ideal torquewaveform, are major technical challenges associated with the design andmanufacture of HHP electric machines. The effects of rotor losses andslot-order space harmonics can significantly complicate the HHP electricmachine design, resulting in more expensive and more complicated statorstacks, or the need of additional filter elements.

For example, the split stator design of the HHP electric machine, notedabove, typically has the two stator halves aligned axially. Rotationallyoffsetting the two stator halves by one half of a stator slot pitch willdecouple the stator slot-order harmonic flux. This approach, however,will still impart a dynamic axial load at the slot-order frequency, inaddition to compounding the difficulty of the insertion of the armaturewinding.

III. SUMMARY

Embodiments of the present disclosure provide methods and systems forreducing, or eliminating, coupling of flux harmonics between the rotorand stator of a HHP. In particular, various embodiments provide ahelical skew, for example, of one full stator slot pitch in oppositedirections in each of the stator or rotor sections, respectively. Suchan exemplary technique can eliminate axial force components, reducingthe expense and complication of stator stack designs.

In one exemplary embodiment, a method is provided for implementingskewing in a hybrid homopolar generator. The method includes aligningmagnets within an axial front segment of a rotor, with correspondingmagnets within an axial back segment of the rotor. The method alsoincludes moving, during assembly, the axial front segment and the axialback segment relative to each other such that magnets in the axial frontsegment and the axial back segment form a pattern.

The illustrious embodiments provide significant technical advantagesover conventional approaches. For example, in the embodiments a majorcomponent of rotor losses is significantly reduced. The opposite skewdirections between the stator opposite a north rotor section and thestator opposite a south rotor section nearly completely decouple theharmonics noted above. Stators designed in accordance with this approachcan enable achieving output voltage requirements and other typicalspecification limits for resulting AC waveform quality. This AC waveformquality can be achieved without adding additional filter elements. Theaxially split stator construction of the HHP provides a solution torotor-stator flux decoupling not normally easily attainable.

Additional features, modes of operations, advantages, and other aspectsof various embodiments are described below with reference to theaccompanying drawings. It is noted that the present disclosure is notlimited to the specific embodiments described herein. These embodimentsare presented for illustrative purposes only. Additional embodiments, ormodifications of the embodiments disclosed, will be readily apparent topersons skilled in the relevant art(s) based on the teachings provided.

IV. BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments may take form in various components andarrangements of components. Illustrative embodiments are shown in theaccompanying drawings, throughout which like reference numerals mayindicate corresponding or similar parts in the various drawings. Thedrawings are only for purposes of illustrating the embodiments and arenot to be construed as limiting the disclosure. Given the followingenabling description of the drawings, the novel aspects of the presentdisclosure should become evident to a person of ordinary skill in therelevant art(s).

FIG. 1 illustrates a hybrid homopolar (HHP) electric generatorconstructed in accordance with various aspects described herein.

FIG. 2 illustrates a stator assembly used in the HHP electric generatordepicted in FIG. 1.

FIG. 3 illustrates more detailed aspects of an exemplary rotor assemblydepicted in FIG. 1.

FIG. 4 illustrates an exemplary stator assembly of an HHP electricgenerator, implementing skewing in accordance with various aspectsdescribed herein.

FIG. 5 illustrates an exemplary herringbone pattern implemented invarious aspects described herein.

FIG. 6 illustrates an exemplary rotor assembly of an HHP electricgenerator, implementing skewing in accordance with various aspectsdescribed herein.

FIG. 7A is an exemplary method of implementing skewing in a stator inaccordance with the various aspects.

FIG. 7B is an exemplary method of implementing skewing in a rotor inaccordance with the various aspects.

FIG. 8A is an example illustration of a graph of a desirable armaturevoltage waveform produced by a rotor rotation in an HHP electricmachine, in accordance with various aspects described herein.

FIG. 8B is an example illustration of a graph of an actual armaturevoltage waveform produced by rotor rotation in typical HHP electricmachines.

FIG. 9A is an example illustration of a graph depicting an ideal torquewaveform produced by rotor and stator interaction during operation of anHHP electric machine, constructed in accordance with the various aspectsdescribed herein.

FIG. 9B is an example illustration of a graph depicting harmonicdistortions in a torque waveform produced by rotor and statorinteraction during operation of typical HHP electric machines.

V. DETAILED DESCRIPTION

While the illustrative embodiments are described herein for particularapplications, it should be understood that the present disclosure is notlimited thereto. Those skilled in the art and with access to theteachings provided herein will recognize additional applications,modifications, and embodiments within the scope thereof and additionalfields in which the present disclosure would be of significant utility.

FIG. 1 illustrates an exemplary hybrid homopolar (HHP) electricgenerator 100 constructed in accordance with various aspects describedherein. The generator 100 includes a stator yoke 102 for holdingarmature coils 103 (see FIG. 2).

FIG. 2 illustrates a partial stator assembly (i.e. stator) 200,including substantially identical stator stacks 202 a and 202 b havingthe armature coils 103 inserted into the stator 200.

Returning to FIG. 1, the generator 100 also includes magnets 104, and arotor 105 formed of several integrated components, discussed more fullybelow. For exemplary magnets 104 can be formed of samarium cobalt.Additionally or alternatively, however, the magnets can be constructedof other suitable materials.

The rotor 105 includes a rotor shaft 106, representing an axis (A)around which major components of the rotor 105 rotate. A rotor hub 108is positioned against the rotor shaft 106. A rotor sleeve 110 isprovided as a centrifugal restraint, positioning the magnets 104 inplace. A field coil 112, for producing an electromagnetic field, isprovided affixed to the stator 200. The field coil 112 iscircumferentially wound into a space in between stator stacks 200 a and200 b. By way of background, the field coils being affixed to therotors, in conventional synchronous electric machines, contribute todifficulty in balancing the rotors. Along with creation of heat by thefield coils, having the field coils attached to the rotor makesconventional synchronous electric machines generally more challenging todesign and construct.

The HHP electric generator 100 also includes a stator core 114. Thestator core 114 conducts flux, produced by the armature winding 103 andthe field coil 112, around the rotor 105 and the stator 200. Inductorpoles 116 further enhance the conduction of the flux produced by thefield coil 112, around the armature winding 103, the stator core 114,stator yoke 102, and the rotor hub 108. As the rotor 105 rotates, theperipherally segmented nature of the inductor poles 116 impartsessential alternating flux variation to the armature winding 103 fromthe unidirectional nature of flux produced from the field coil 112. Theexemplary inductor poles 116, depicted in FIG. 1, can be constructed oflaminated silicon steel.

FIG. 3 provides a more detailed illustration of the rotor 105, includingthe inductor poles 116. Additionally, as the rotor 105 rotates, theperipherally segmented nature of the magnets 104 imparts essentialalternating flux variation to the armature winding 103. The alternatingflux produced from the field coil 112 and inductor poles 116 can add orsubtract to the alternating flux produced from the magnets 104. Thisadding or subtracting occurs according to the direction and magnitude ofthe direct current (DC) current supplied to the field coil 112.

FIG. 4 illustrates an exemplary stator assembly 400 for an HHP electricgenerator, implementing skewing in accordance with various aspectsdescribed herein. The stator assembly 400 includes an axial front stator401, an axial back stator 402, and stator teeth 403. More specifically,FIG. 4 is an illustration of skewing implemented in the stator 400 inaccordance with the various aspects. The under-laying computer-aideddesign (CAD) illustration of FIG. 4 is that of un-skewed stators. Thesuperimposed dotted lines illustrate how all edges are to be transformedinto the skewing embodiment, described herein.

The axial front stator 401 includes a front edge 401 a and a back edge401 b. Similarly, the axial back stator 402 includes a front edge 402 aand a back edge 402 b. Skewing functionality is implemented bytransforming the respective front and back stators 401 and 402 to form askew pattern 404 (see FIG. 5), generally resembling a herringbone. FIG.5 illustrates a generic herringbone pattern 500.

By way of background, and as understood by those of skill in the art,the rotor 105 rotates inside of the stator 200. The rotor 105 is rotatedby a mechanical power source, such as from gearing from an aircraftengine. Electric currents in the armature winding 103 react with themagnetic fields produced by the rotor 105 (inductor poles 116interacting with the field coil 112, and the magnets 104), to provideelectrical power to an electrical load. In a conventional HHP electricgenerator, the magnetic field produced by the armature winding 103rotates in a manner to maintain alignment with the magnetic fieldproduced by the rotor 105.

As a consequence torque ripple, discussed more fully below, the rotor105 actually turns in a non-uniform motion, having superimposed smallsteps or jerks. By building the rotor 105, or the stator 200, with atwist (i.e., skewing the design), the magnets 104 and inductor poles 116(or edges of teeth in the stator 200) are twisted around the rotor shaft106. This skewing causes a reciprocal effect of the magnetic field,cancelling the torque ripple, ultimately resulting in smoother rotationof the rotor 105 and more efficient operation of the HHP electricgenerator 100. FIG. 4 illustrates this skewing process in a statorassembly 400, in accordance with an embodiment.

In FIG. 4, an un-skewed trajectory edge 406 a (dotted line) is shownwith reference to a stator tooth 403 a (of teeth 403) of the stator 400.By way of example, the edge 406 a can be peripherally shifted duringassembly, such as during the lamination stacking process. During thisskewing, or shifting process, the edge 406 a travels to form a shiftededge 406 b at a skew angle (θ) 412. Arrows indicate movement of theun-skewed edge 406 a forming the skewed edge 406 b.

Skewing is implemented across all of the stator teeth 403 for respectivefront and back stators 401 and 402, to form a herringbone pattern atbracket 404. See FIG. 5, depicting a more detailed illustration of aherringbone pattern 500. In FIG. 4, the bracket 404 depicts exampleskews 414 and 416 associated with outer surfaces of the stator stacks401 and 402. The amount (degree) of skewing is a function of a thenumber of teeth 403 of the stators 401 and 402 in the stator assembly400.

FIG. 5, for purposes of illustration, depict skews 414 and 416superimposed over the generic herringbone pattern 500. Following apattern resembling the herringbone pattern 500 enables the armaturewinding 103 to be more easily assembled while implementing the skewfunctionality.

Returning to FIG. 4, the skew of the axial front stator 401 is shiftedclockwise to form the exemplary skew 414. The skew of the axial backstator 402 is shifted counterclockwise to form the exemplary skew 416.In practice, to implement skewing, the front stator 401 is twisted in asubstantially clockwise direction and the back stator 402 is twisted ina substantially counterclockwise direction.

The twisting occurs as a function of the slot-pitch dimension 413 of thestator teeth 403 such that the axial front stator 401, the twistingtravels clockwise one slot-pitch dimension going from front edge 401 ato a back edge 401 b. Likewise, in the axial back stator 402, thetwisting travels counter-clockwise one slot-pitch dimension going from afront edge 402 a to a back edge 402 b. For facilitating insertion of thearmature winding 103, edges of the teeth 403 located at the back edge401 b should peripherally align with edges located at the front edge 402a, of corresponding teeth, across an inter-stack space 417.

As the assembly continues, the armature winding 103 is insertedbeginning at the back edge 401 b of the axial front stator 401(clockwise) to the front edge 402 a of the axial back stator 402(counterclockwise). During this process, slot openings, such as slotopenings 418 a and 418 b at respective edge 401 b and 402 a, aredesirably aligned as depicted along alignment line (L) of FIG. 4.

FIG. 6. illustrates an exemplary rotor assembly 600 for an HHP electricgenerator implementing skewing in accordance with various aspectsdescribed herein. In the embodiments, skewing can be implemented in HHPelectric generators in a stator assembly, or in a rotor assembly.

The rotor 600 includes inductor poles 602 a/602 b alternating withmagnets 604 a/604 b. The rotor 600 also includes an axial front rotor606 and an axial back rotor 608. Similar to the construction of thestator 400, the axial front rotor 606 includes a front edge 606 a and aback edge 606 b. The axial back rotor 608 includes a front edge 608 aand a back edge 608 b.

The exemplary rotor 600 skews rotor inductor poles, such as the inductorpoles 602 a, one slot pitch 610 over an axial length 605 a of the frontrotor 606. Correspondingly, the inductor poles 602 b is skewed one slotpitch 610 over an axial length 605 b of the axial back rotor 608. Arrowsindicate movement of an un-skewed edge 612 a forming a skewed edge 612 bin the inductor pole 602 a of the axial front rotor 606. A skewed edge614 is depicted in the inductor pole 602 b of the axial back rotor 608.The skewing of inductor poles in the axial front rotor 606 relative tothe axial back rotor 608 occurs in a pattern, similar to skewing in thestator assembly 400, discussed above. This pattern can also resemble theherringbone pattern 500 of FIG. 5.

In the rotor 600, skewing is implemented with a clockwise twisttravelling the one slot pitch 610 from front edge 606 a to back edge 606b of the axial front rotor 606. In the axial back rotor 608, skewing isimplemented with a counter-clockwise twist travelling one slot-pitch 610from front edge 608 a to back edge 608 b.

Skewing, as described herein, can be implemented in concert withoptimization of inductor poles arcs (not shown) and stator slot openingsize. Skewing can also be implemented with holes for flux shaping inlaminations at the edges 606 a/b and 608 a/b of respective inductorpoles 602 a and 602 b, and in shaping of inductor poles faces and/orbridges, in embodiments utilizing embedded inductor poles designs. In analternative embodiment, each axial front rotor and axial back rotor canbe fabricated in two identical sections then assembled such that thesections are rotated by one-half slot pitch. When the axial front rotorand axial back rotor are aligned, the two most innermost sectionsdesirably have the same rotation direction.

FIG. 7A is an exemplary method 700 of practicing an embodiment of thepresent disclosure for implementing skewing in a hybrid homopolarelectric machine, the machine including a stator. The method 700includes aligning slots within an axial front segment of the stator withcorresponding slots within an axial back segment of the stator in block702. The method also includes twisting the axial front segment in asubstantially clockwise direction by a predetermined amount and twistingthe axial back segment in a substantially counterclockwise direction bythe predetermined amount, as depicted in block 704. The twisting forms apattern in respective components of the axial front segment and theaxial back segment, as depicted in block 706.

FIG. 7B is an exemplary method 708 of practicing an embodiment of thepresent disclosure for implementing skewing in a hybrid homopolarelectric machine, the machine including a rotor. The method 708 includesaligning inductor poles within an axial front segment of the rotor withcorresponding inductor poles within an axial back segment of the rotorin block 710. The method also includes twisting the axial front segmentin a substantially clockwise direction by a predetermined amount andtwisting the axial back segment in a substantially counterclockwisedirection by the predetermined amount, as depicted in block 712. Thetwisting forms a pattern in respective components of the axial frontsegment and the axial back segment, as depicted in block 714.

FIGS. 8A, 8B, 9A, and 9B graphically depict performance improvementsprovided by the various aspects described herein.

For example, FIG. 8A illustrates a graph 800 of a desirable, perfectlysinusoidal, voltage waveform 802 produced as a rotor in an HHP electricgenerator rotates. The waveform 802 is graphed along voltage axes 804and rotation position axes (i.e., motor electrical cycle) 806. Theperfectly sinusoidal voltage waveform 802 is not achievable usingconventional HHP electric generators.

FIG. 8B illustrates a graph 808 of an actual voltage waveform 810produced as a rotor in a conventional HHP electric generator rotates.The waveform 810 includes harmonic ripples 812 (voltage variations) thatcan cause losses and heating in the load, armature winding, and rotor,as the rotor rotates. The higher the order and magnitude of the harmonicripples 812, the greater the losses in the load and armature winding,and the heating in the rotor. These effects can significantly reducegenerator performance. Torque waveforms in conventional HHP electricgenerators include similar effects.

FIG. 9A is a graph 900 of a relatively flat, ideal torque waveform 902produced during operation of an HHP electric generator.

In contrast, FIG. 9B illustrates a graph 904 depicting harmonicdistortions similar to the graph 808 in FIG. 8B. The graph 904 depicts atorque waveform 906 produced in a conventional HHP electric generator.The torque waveform 906 includes an imposition of high order harmonics908, which can create excessive vibration and noise. These effects alsorepresent considerations in generator bearing design. The harmonics 908,also known as slot-order space harmonics, are caused by the magneticreluctance variations between slot openings in a stator of aconventional HHP electric generator. The harmonics 908 are oftenreinforced by a pulse-count of time-harmonic passive rectificationswitching and are a main contributor to the losses.

By implementing skewing either on the stator 400 or on the rotor 600,the various aspects described herein, reduce or eliminate the harmonicripples 812 in the armature voltage waveform 810, depicted in FIG. 8B.That is, an armature voltage waveform produced by the stator 400 or therotor 600 will more closely resemble the pure sinusoidal waveform 802.Similarly, various aspects described herein reduce or eliminate theslot-order space harmonics 908 in the torque waveform 906 depicted inFIG. 9B. By contrast, a torque waveform produced by the stator 400 orthe rotor 600 will more closely resemble the substantially flat torquewaveform 902 of FIG. 9A.

Alternative embodiments, examples, and modifications which would stillbe encompassed by the disclosure may be made by those skilled in theart, particularly in light of the foregoing teachings. Further, itshould be understood that the terminology used to describe thedisclosure in intended to be in the nature of words of descriptionrather than of limitation.

Those skilled in the relevant art(s) will appreciate that variousadaptations and modifications of various aspects described hereindescribed above can be configured without departing from the scope andspirit of the disclosure. Therefore, it is to be understood that, withinthe scope of the appended claims, the disclosure may be practiced otherthan as specifically described herein.

What is claimed is:
 1. A method for implementing skewing in a hybridhomopolar generator comprising: aligning inductor pole inductor poleswithin an axial front segment of a rotor, with corresponding inductorpoles inductor pole within an axial back segment of the rotor; andmoving, during assembly, the axial front segment and the axial backsegment relative to each other such that inductor poles in the axialfront segment and the axial back segment form a pattern.
 2. The methodof claim 1, wherein inductor poles within the axial front segment andthe axial back segment of the rotor are moved.
 3. The method of claim 1,wherein the pattern resembles a herringbone shape.
 4. The method ofclaim 3, wherein the moving includes twisting the axial front segmentrelative to the axial back segment by at least one stator slot pitch. 5.The method of claim 4, wherein the axial front segment is twisted in onedirection and the axial back segment is twisted in another direction. 6.The method of claim 5, wherein the axial front segment is twistedclockwise, and the axial back segment is twisted counterclockwise.
 7. Amethod for implementing skewing in a hybrid homopolar electric machineincluding a least a stator and an armature winding for insertion withinthe stator, the method comprising: aligning inductor poles within anaxial front segment of the rotor with corresponding maintenance withinan axial back segment of the rotor; and twisting the axial front segmentin a substantially first direction by a predetermined amount andtwisting the axial back segment in a substantially second direction bythe predetermined amount; wherein the twisting forms a pattern inrespective components of the axial front segment and the axial backsegment.
 8. The method of claim 7, wherein the predetermined amount is afunction of relative positioning of the axial front segment and theaxial back segment.
 9. The method of claim 7, wherein the firstdirection is clockwise and the second direction is counterclockwise. 10.The method of claim 7, wherein the predetermined amount is at least oneslot pitch.
 11. The method of claim 7, wherein the rotor includes aplurality of inductor poles axially shifted by a slot width.
 12. Themethod of claim 7, wherein the aligning occurs during assembly of therotor.
 13. An electric machine including a stator and a rotor,comprising: an axial front segment of the rotor; an axial back segmentof the rotor positioned in a predetermined manner relative to the axialfront segment; and one or more inductor poles of the axial front segmentare positioned relative to one or more inductor poles of the axial backsegment based on a characteristic of the stator.
 14. The electricmachine of claim 13, wherein the machine is a hybrid homopolargenerator.
 15. The electric machine of claim 14, wherein the positionedinductor poles of the axial front segment and the axial back segmentresemble a herringbone pattern.
 16. The electric machine of claim 14,wherein the characteristic of the stator is slot pitch.
 17. The electricmachine of claim 13, wherein the one or more inductor poles of axialfront segment are positioned with respect to the one or more inductorpoles of the axial back segment by at least one stator slot pitch. 18.The electric machine of claim 13, wherein the axial front segment istwisted in a substantially first direction by a predetermined amount andthe axial back segment in a substantially second direction by thepredetermined amount.
 19. The electric machine of claim 18, wherein thefirst direction is clockwise and the second direction iscounterclockwise.
 20. The electric machine of claim 19, wherein thepredetermined amount is at least one slot pitch.